Microelectronic devices are fabricated on semiconductor wafers using a variety of techniques, e.g. including deposition techniques (CVD, PECVD, PVD, etc) and removal techniques (e.g. chemical etching, CMP, etc). Semiconductor e.g. silicon wafers may be further treated in ways that alter their mass distribution e.g. by cleaning, ion implantation, lithography and the like. These treatment techniques typically cause a change in the mass distribution at or on the surface of the semiconductor wafer. The configuration of the changes to the surface are often vital to the functioning of the device, so it is desirable for quality control purposes to assess wafers during production in order to determine whether they have the correct configuration.
A number of existing measurement techniques are known. For etching treatment, one known technique is to break a treated wafer and perform a detailed analysis of its cross-section. If the analysis shows treatment to have been successful, it is assumed that the batch of wafers manufactured at the same time as the tested (broken) wafer also have the correct configuration. The disadvantage of this process is that the broken wafers cannot be used and are therefore wasted, and that the process is typically interrupted (i.e. fabrication stopped) every time a wafer from a batch is tested. Continuous production is therefore not feasible.
Other known measurement techniques depend on the type of treatment or the properties of materials created by the treatment. For example, treated wafers can be measured using ellipsometry when they contain dielectrics or wafers can be tested using resistivity probes when conductive metals are deposited thereon.